Electrical resistance



June 22, I954 G. E. coMsTocK, 3RD 2,681,944

ELECTRICAL RESISTANCE Filed Jan. 15, 1952 2 Sheets-Sheet l II I I Fag. 3V 39 38 Hz: '20 rney June 22, 1954 G. E. COMSTOCK, 3RD 2,681,944

ELECTRICAL RESISTANCE '2 Sheets-Sheet 2 Filed Jan. 15, 1952 Jilln 650%?[Co/ 15 Tack 3 Patented June 22, 1954 ELECTRICAL RESISTANCE George E.Comstock 3rd, Holden, Mass-., assignor to Norton Company, Worcester;Mass-., acorpm ration of Massachusetts Application January 15, 19.52,.SerialjNo. 266,575

(Cl'. 13T25) 9 Claims. 1

The invention relates to electrical resistances especially forheating-furnaces.

One object of the invention is to provide a heati'n'g'resistance capableof use at high temperatures in an oxidizing atmosphere. Another objectis to provide a cold end construction for zirconia resistors greatlyincreasingtheir effectiveness. Another object is to provide-a practicalcombination of electric transformer and zirconia resistor so that therefractoriness and resistance to oxidation. of. this material can betaken advantage of while avoiding detriment from the fact that it has anegative temperature coefficient' of resistance.

Another object of the invention is to provide an electrical heatingresistance that can be operated at very high temperatures and which hasa life expectancy substantially not limited by chemical reactionsbetween the atmosphere and. the resistance.

Other objects will be'in part obvious or in part pointed outhereinafter.

In the accompanying drawings illwstrating one of many possibleembodiments of the mechanical and electrical features of this invention:

Figure 1 is a vertical sectional view of a furnace having electricalresistances according to the invention,

Figure 2 is an elevation of an electrical resistance unit according tothe invention, certain sleeves being shown in section,

Figure 3 is a fragmentary view on an enlarged scale taken on the line 33of Figure 1,

Figure 4 is a plan view of a transformer,

Figure 5 is a sectional view ofthe transformer taken on the line 55 ofFigure 4.

Figure 6 is a wiring diagram.

The resistance unit. which may also be called a resistor, has terminals.of some refractory oxidation resistant metal such as nickel, stainlesssteel, or the well-known nickel-chromium alloy one. variety of which is60 Ni, 24' Fe, 16 Cr and 0.1 C. The outer ends of these terminals willbe cold enoughso that they can be in contact with copper or aluminumconductors or with spring metal clips or the like in order to connectthe resistor in circuit. In order to protect the inner ends of thesemetal terminals from the intense heat liberated by the zirconia resistorrod in which most of the voltagedrop occurs I provide cold ends;connecting the zirconia resistor rod with the metal terminals which arerefractory enough to withstand the heat. liberated by the zirconiaresistor rod. and are conductive; enou h so that they will not overheatby reason of the heat liberated due tothe voltage droptherein, andpreferably I utilize two Oxide materials, one in contact with thezirconia resistor'rod and the other in contact with the metal terminals.For the former or inner of these materials- I use oxide selected fromthe group consisting of thorium dioxide, ThOz, cerie oxide, C'eOz andmixtures thereof and zinc oxide, ZnO; whilefor the latter or outer ofthese materials I use zinc'oxide, ZnO; hence in one embodiment of theinvention both materials are 2110; In order to keep the oxideor oxidesfrom fracturing I use it or them in powder form and hold it or them inplace with refractory sleeves, preferably zirconia sleeves. Doubtless in'the course of use the powder sinters somewhat but that isof noconsequence as it works equally well as powderorsinteredmaterial and if,once having sintered; it'- laterfractures, that is of no consequenceeither: I provide spring pressure to hold the rods against theoxide. Ina preferred form of the inventionI place discrete portions of thoriumdioxide-powder-and' zincoxide powder in the sleeves, the former'incontact with the zirconia resistors, the latter in contact with themetal rods. Thus I achieve a successive step down from the-hightemperature of the zirconia rods, to the veryrefractory thoria (thoriumdioxide) to the less refractory zinc oxide to the metal terminalrodswhich, of course, are subject to oxidation but whose resistivityisso'low they are heated. practically only by conduction and their endsexposed to the-atmosphere are at such low temperatures that they can beused for years without deterioration.

Another feature of the invention is theuse of a constant currenttransformer of simple design so that the negative temperaturecoefficient of the zirconia resistors is no detriment at all. However aball'astingresistance or a servo-controlled motor" driven variabletransformer can be substituted for the simple constant currenttransformer in certain embodiments ofthe invention.

Referring now to Figures 1 and 2, I provide rods orbars Hlof'sinteredzirconia, that is to say made by pressing and' sintering (eithersimultaneously or' not) comminuted zirconia without any significantamount of binder except temporary binder. If desired a massof zirconiaparticles mixed with a little temporary binder such as a solution ofdextrine can beplaced in a rubber mold having' an elongated cylindricalmelding space and the mass can be pressed by hydraulic pressure inamannernow' well known in the art. Subsequentlythe pressedi rods can befired to sinterthe zirconia particles together in a manner also wellknown in the art. While I can use zirconia relatively free fromimpurities or addition agents I prefer to use the stabilized zirconiacontaining from 3% to 6% lime which is fully described in U. S. LettersPatent No. 2,535,526 granted on application of Ballard and MarshallDecember 26, 1950. This is because zirconia stabilized with lime asdescribed in this patent does not have the abnormal and eccentricexpansion behavior of unstabilized zirconia. Stabilized zirconiadescribed in the aforesaid patent is predominantly cubic in crystallinestructure whereas unstabilized zirconia having no lime or otherstabilizing agent is predominantly of monoclinic crystal structure. Iintend to include any content of hafnia under the designation zirconiabecause hafnia is so similar to zirconia in its chemical and physicalproperties as to be substantially indistinguishable therefrom except byX-ray analysis and because available zirconia ores contain a minorpercentage of hafnia which, for most purposes including the manufactureof resistors, is not separated from the zirconia. Indeed it would bevery expensive and highly impractical to try to remove the content ofhafnia from the zirconia oreor from the purified zirconia obtainedtherefrom. Since hafnia is chemically so much like zirconia there is nota great deal of data on how much hafnia to expect in a given lot ofzirconia or ore but it is known that the hafnia is always in minorproportion. So far as this invention is concerned the hafnia might aswell be zirconia and it is not to be considered to be an impurity.I-Iafnia likewise has a monoclinic form and a cubic form and themonoclinic hafnia passes through the phase change or inversion and hasthe abnormal and eccentric expansion behavior of the monoclinic zirconiawhereas hafnia stabilized with 3% to 6% of lime which crystallizes inthe cubic system does not pass through the phase change or inversion anddoes not have the abnormal or eccentric expansion behavior of themonoclinic hafnia.

I provide a pair of zirconia sleeves II for each rod or bar Ill, andthese zirconia sleeves are advantageously made out of the same zirconiaas is used for making the rods II! and the sleeves can be manufacturedin the same way as the rods I0, using a steel core in the rubber mold toform the bore. In the preferred form of my invention each sleeve I Icontains a quantity of thoria powder I2 and a quantity of zinc oxidepowder I3, the former adjacent the rods or bar In which project into thesleeves and the latter adjacent metal rods which project into thesleeves II. The rods I4 constitute the terminals hereinbefore referredto.

The furnace can be of any desired shape or type and my invention is inno way limited thereto. Merely as an illustrative example of the use ofmp resistances I disclose a steel box I5 of rectangular parallelepipedalshape supported on steel supports I5 and lined with bottom bricks 11,side wall bricks 18, end wall bricks I9, large top slabs 20 and topbricks 2|, all of the bricks and slabs being made of refractorymaterial. The furnace may have a ware support in the form of a fiat slab22 of refractor material with legs 23 so that one of the rods In can belocated under the slab 22. For access to the inside of the furnace thebox I5 has an open side and bricks I8 can be removed.

In order to provide the spring pressure against the powders in thesleeves I can adopt any suitable expedient and the following mechanicalarrangement is illustrative: steel brackets 24 are secured to theoutside of the box I5 at the top and bottom thereof and at One end ofthe furnace steel bars 25 are bolted to the brackets 24 by means ofbolts 26 and nuts 2'! while at the other end of the furnace flat springsteel bars 28 are secured to the brackets 24 by means of screws 29. Thesteel bars 25 are rigid and have insulating pieces as at the free endsthereof against which the metal rods I4 bear; the insulating pieces 30can be made of asbestos, alumina, mica or any other electricalinsulating material which will not be afiected by the moderate amount ofheat delivered by the ends of the metal rods I4. The spring steel bars23 have similar insulating pieces 3| at the free ends thereof which bearagainst the metal rods I4; thus compression is exerted by the rods I4and the rods III upon the powder I2 and the powder I3 in all of thezirconia sleeves I I.

The electrical connections may be made in any desired way. As shown astud 32 is Welded to the outside of the box I5 and supports aninsulating plate 33 (which can be made of hard rubber) secured in placeby nuts 34 and through the far ends of the plate 33 extend binding posts35 havin nuts 36 connecting incoming conductors 31 to braided metalribbon conductors 38. Referring now to Figure 3, the braided metalribbon conductors 38 are electrically connected to the metal rods I4 bymeans of spring metal clips 39. At the other end of the furnace theother two rods I4 are connected together by a braided metal ribbonconductor 4|! and spring metal clips 39.

Figures 4 and 5 illustrate a simple and inexpensive transformer thesecondary 4| of which has a substantially constant current output ratherthan a substantially constant voltage output. The core of thistransformer consists of a central rectangular parallelepiped 42 of ironand a pair of outside rectangular parallelepipeds 43 01' iron, a topyoke 44 of iron and a bottom yoke 45 of iron, these parts being heldtogether by screws such as the screws 46.

The secondary 4| is connected to cords 41 and 48 which pass over singlepulleys 49 and 50 supported by brackets 5| and 52 connected to the topyoke 44; the cords then pass over a double pulley 53 and extenddownwardly where they join under a ring 54; in other words a single cordcan be used the two ends of which are designated 41 and 48. A hook 55which can be suspended from the ring 54 has a scale pan 55 upon whichslotted weights 51, 58 and 59 can be placed.

The transformer has a primary 60 which rests upon the bottom yoke 45 andwhich may be secured from accidental displacement in any manner, notshown. The wiring is extremely simple and is illustrated in Figure 6,wherein the primary connections 6| supply alternating current to theprimary 60, and the secondary connections 31 are connected to theresistor rods or bars I0 in the manner already described, the connectionbetween these rods I0 including the braided metal ribbon conductor 40.

Whenever the sum of the ohmic resistance of the rods Ill decreases whichhappens when the temperature thereof rises, more current flows throughthe secondary 4| which increases the repulsion between the primary B0and the secondary 4| and the latter therefore moves upwardly. Theweights 51, 58 and 59 counter- For any particular ohmic resistance ofthe tworods [0 taken together the secondary II will find a particularlevel and so sensitive can this transformer'bemade by selecti'ng theweights 51 58- and 59 properly, that is to say very nearly tocounterbalance thesecondary 4|, that the residual variation in currentis'verysmalll Material the ohmic resistance of which decreases when thetemperature thereof rises is said to have anegative temperaturecoefficient of resistance. This is characteristic of the rods i0, thatis to say zirconia of all varieties described herein has a negativetemperature coefficient ofresistance.

In. order to obtain data for calculating the parameters'of circuits-tobe used in embodiments of. my invention I performed a number ofexperiments. Iformed a rod" of sintered. stabilized zirconia. two incheslong and one-quarter inch inv diameter. and provided sleeves H of. thesame material into. which the rod projected and provided nickel terminalrods [4' also projecting intothe zirconia sleeves the remaining space inwhich was filled with zinc oxide powder. Spring pressure held the zincoxide powder under compression in the. zirconia sleeves. This unit wasrun for many hours in the open air that is to say notinan enclosure andexhibited the characteristics. indicated inTable I.

Table I In Table I- the temperatures indicated may be as much as 100 C.or 200 C. lower than the actual" surface temperatures of the zirconiarod because of the low emissivity of this material. Thoria powder aswell as zinc oxide powder should be: used when. prolonged operationthatis, more than a few hours--at: the higher junction temperatures isdesired, since zinc oxide is not adequately stable for continuousoperation when its temperature in any region is higher thanapproximately 1300 C. Table II shows the resistivities of zinc oxide andTable III shows the resistivities of thoria at the various temperatureslisted.

Table II Resistivity of ZnO, ohm-cm.

Temperature, Degrees Centigrade Table III Resistivity of 90% Thoria,Ceria, ohm-cm.

Temperature, Degrees Centigrade 6 with some-outside source. of heat,such, as: an auxiliary electric heater orwith a flame. Silicon carbideresistors parallel to the rods I'll can be used and then withdrawn afterthe rods [0 are capable of carrying the load. It is very easy, however,toheat a furnace such as shown in Figure 1 with a gas flame which can beturned ofi whenever the rods 10'- are able to carry the load.

As an indication of the varying resistivity of the stabilized zirconiareferred to herein at various temperatures the following values weredetermined experimentally.

Table IV Refsilsltivity 0 me True Temperature, Degrees- Centigrade Ofthe oxide materials mentioned thorium dioxide, ThOz,isveryrefractory,having a melting point as. given in handbooks of greater than 2800 C.and it has a lower resistivity than zirconia at working temperaturesand. hence the heat-liberated therein will be less per unit length (samecross sectionassumed) at any given working temperature. Ceric oxide,0e02, is likewise refractory, having, a. melting, point as given inhandbooks of about 1950 C. and it likewise has a. lowerresistivity thanzirconia at working temperatures. Mixtures of these oxides in allproportions are compatible in: this invention and neither one of themnor any mixture thereof reacts with zirconia. to any detrimental extentsofar asI have observed or am aware. I do not want. itto be understood.that impurities cannot be: presentinz fact I use readily availablepowders which certainly contain impurities which are probably beneficialas by increasing the conductivity. However it can be said that thepowders are; commercially speaking, selected from the group. aforesaid.

Zine oxide, ZnO; which can be part or all of the cold end, is refractoryand has a melting point as given in the handbooks of greater than 1890C. and has a resistivity which is lower than that of thorium dioxide. Itdoes not detrimentally react with zirconia nor with thoria nor withceric oxide so far as I have observed or am aware at least when used asherein described but I do not use mixtures of zinc oxide with either orboth of the other two oxides. The zinc oxide can also have impuritiestherein which are probably beneficial, and so long as commerciallyspeaking the material i zinc oxide, ZnO, it is satisfactory in thisinvention.

The zirconia sleeves II have important functions besides that of holdingthe oxide powder or powders. They conduct more current and hence lowerthe resistivity per unit length of the sleeves as compared with the rodsID by reason of the increased cross section and furthermore they delivercurrent to the rods [0 at the cylindrical surface thereof which appearsto prevent channeling of the rods. Because of the negative temperaturecoefficient of resistance of zirconia there is a tendency for thecurrent to flow chiefly in a narrow path around the axis of the rod orbar made thereof because this part is thermally insulated and thereforegets hotter and hotter since the hotter it gets the more current flowstherethrough until finally such central path melts or disintegrates. Butby providing sleeves which deliver some current to the periphery of therods it I appear to have prevented channeling at least in practicalembodiments of this invention.

The zirconia of the rods ill need not be pure either and can be definedas at least 97% pure zirconium oxide and hafnium oxide and lime if any;in other words it can have 3% of impurities and neither the hafniumoxide (hafnia) nor the lime are impurities except that the latter shouldnot be over 6% of the total of zirconium oxide and hafnium oxide. Byzirconium oxide and zirconia I mean ZrOz and by hafnium oxide I meanHfDz but the latter is calculated as ZrOg. In commerce HfOz is reportedas 2102 when included therein and U. S. Patent No. 2,535,526 treats thehafnium oxide content as zirconia.

In the constant current transformer of Fi ures t and the weight of thesecondary coil 4| minus the balancing weights 51, 58 and 59 constitutes,mean urging the secondary coil towards the primary coil 2-9. For gravityspring pressure, pneumatic pressure or hydraulic pressure could besubstituted. If a spring is used it should preferably b compensated; seeU. S. Patent No, 1,393,758. Against this means urging the secondary coilis opposted the repulsion which coils carrying alternating current outof phase, as when one coil induces current in the other, exert on eachother.

It will thus be seen that there has been provided by this inventionelectrical resistances or resistors in which the various objectshereinabove set forth together with many thoroughly practical advantagesare successfully achieved. As various possible embodiments may be madeof the above invention and as many changes might be made in theembodiments above set forth, it is to be understood that all matterhereinbefore set forth or shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

Iclaim:

1. An electrical resistance comprisin a rod of zirconia and a pair ofends therefor made of oxide selected from the group consisting ofthorium dioxide, ceric oxide and mixtures thereof and zinc oxide.

2. An electrical resistance comprising a rod of zircoma, thorium dioxidein contact with the ends of said rod, and zinc oxide in contact with thethorium oxide.

3. An electrical resistance according to claim 1 having a. pair of zincoxide ends outside of and in contact with said ends made of oxideselected from the aforesaid group.

4. An electrical resistance according to claim 3 having a pair ofzirconia sleeves fitting the ends of and surrounding said rod at theends thereof, said pair of ends of oxide selected from the aforesaidgroup and said pair of zinc oxide ends being in said sleeves.

5. An electrical resistance according to claim 4 in which the oxideselected from the aforesaid group and the zinc oxide are powder thereof.

6. An electrical resistance according to claim 1 having a pair ofzirconia sleeves fitting the ends of and surrounding said rod at theends thereof, said pair of ends of oxide selected from the aforesaidgroup being in said sleeves.

7. An electrical resistance according to claim 6 in which the oxideselected from the aforesaid group is powder thereof.

8. An electrical resistance according to claim 1 in which the oxideselected from the aforesaid group is powder thereof.

9. An electrical resistance according to claim 8 having a pair of zincoxide ends outside of and in contact with said ends made of oxideselected from the aforesaid group.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 729,812 Troy June 2, 1903 788,493 Parker Apr. 25, 19051,368,507 Kiekstad Feb. 15, 1921 1,393,758 Crompton, Jr. Oct. 18, 19211,470,195 DeRoiboul Oct. 29, 1923 1,613,877 Dyckerhof Jan. 11, 19271,742,286 Shaw Jan. 7, 1930 2,516,570 Hartwig et a1. July 25, 19502,535,526 Ballard et a1. Dec. 26, 1950

1. AN ELECTRICAL RESISTANCE COMPRISING A ROD OF ZIRCONIA AND A PAIR OFENDS THEREFOR MADE OF OXIDE SELECTED FROM THE GROUP CONSISTING OFTHORIUM DIOXIDE, CERIC OXIDE AND MIXTURES THEREOF AND ZINC OXIDE.